1. Field of the Invention
This invention relates to high-range, wide-angle zoom lenses of compact size suited to be used in lens shutter cameras or video cameras and, more particularly, to zoom lenses having an angular field widened while still permitting the total length (the distance from the front vertex to the image plane) to be shortened for excellent portability.
2. Description of the Related Art
Recently, the art of leaf shutter cameras, or video cameras, has shown a tendency to reduce the size of the camera. Along with this, there is a growing demand for compact zoom lenses of short total length.
Particularly, the leaf shutter camera, owing to the advancement of technology of the peripheral devices such as an electric circuit for driving the zooming lens members, is getting smaller and smaller in size. The photographic lens with which such a camera is equipped, too, is required to be compact despite the high range as it is of the zoom type.
In the past, for the zoom lenses for the leaf shutter cameras, the use of two lens units of positive and negative refractive powers in total, or the so-called 2-unit type constituted the mainstream. Since this 2-unit zoom lens is simple in construction and easy to operate, the minimization of the size of the camera is facilitated, the production cost is relatively low and other advantages are gained.
However, because only one lens unit bears all the function of varying the focal length, the zoom ratio is as low as 1.6 to 2. Much increase of the zoom ratio calls for a rapid increase of the size of the lens system. At the same time, the difficulty of maintaining good stability of high optical performance increases greatly.
Given the 2-unit zoom lens as a base, its first lens unit may be divided into two parts of positive refractive powers. Thus, the three lens units of positive, positive and negative refractive powers constitute the entire system with an aim at obtaining an ever higher range. Such a 3-unit zoom lens is proposed in, for example, Japanese Laid-Open Patent Applications Nos. Hei 3-282409, Hei 4-37810 and Hei 4-76511.
However, with this type in use, when the angular field is widened to 35.degree. or more in the semiangle, the distance that the entrance pupil moves with zooming gets longer. For this reason, to increase the zoom ratio, a difficult problem of suppressing the variation of aberrations with zooming to a minimum is encountered.
Besides this, there is another technique that makes use of an even larger number of lens units. By employing this technique, the semiangle of field for the wide-angle end can be widened to 38.degree. or thereabout and the range can be increased to 3.5 or thereabout. Such a wide-angle, high-range zoom lens is proposed in, for example, Japanese Laid-Open Patent Applications Nos. Hei 2-72316 and Hei 3-249614.
However, these zoom lenses of the character described above suffer large increases both of the diameter of the front lens members and of the total length of the entire system, and are, therefore, not always sufficiently amenable to a photographic lens for a compact camera.
Particularly, in application to the camera using the external finder, a problem arises in that, in wide-angle settings, the lens barrel vignettes the field of view of the finder. To avoid this, an alternative problem is encountered in that a limitation has to be imposed on the finder arrangement and the form of the camera.
In general, zoom lenses have a property that the stronger the refractive power of each of the lens units, the shorter the required movement of each of the lens units for the predetermined zoom ratio becomes. This means that the zoom ratio can be increased in such a manner that the total length of the entire system is shortened. However, if the refractive power of each of the lens units is merely strengthened, the aberrations vary to greater extent with zooming. Particularly, when the requirements of increasing the zoom ratio and of widening the angular field are fulfilled at once, there arises a difficult problem of obtaining good optical performance through the entire zooming range.